Spike will hold Noctis to that IOU. That's a form of currency here that's more valuable than other things. Mind, when Noctis starts summoning weapons there's a startled "Oi!" yelped out in protest. Ardyn is free to move to dodge (as Spike could indeed tell the other was letting him be held), but Spike is keeping in close to him. A hand or something not far the whole time to avoid letting him slip away.

"You could kill someone!" Spike scolds Noctis. Which the dark haired man my want, but Spike wanted to fight Ardyn. That's why he intervened. It's why he's staying close to Ardyn now so the other can't slip away.